Muscle activation increases tissue stiffness as measured by elastography. We applied ultrasound time‑harmonic elastography (THE) to the vastus lateralis muscle during passive tonic vibration reflex (TVR) and active voluntary contraction (VC) to test whether TVR can serve as a stimulation method for functional assessment of skeletal muscle stiffness. Twenty‑five asymptomatic volunteers (8 females, mean age 34 ± 8 years) underwent five consecutive THE examinations at three VC loads (15, 22, and 37 N) and during TVR stimulation with 100 Hz frequency and 200 µm (low) and 400 µm (moderate) amplitude. Using standard line‑by‑line ultrasound, THE acquired induced shear waves of 60, 70, and 80 Hz frequency with a frame rate of 100 Hz. Shear wave speed (SWS) was reconstructed as a proxy for muscle stiffness and statistically analyzed. SWS increased significantly from 1.66 ± 0.11 m/s at rest to 1.79 ± 0.12 m/s, 1.93 ± 0.13 m/s, and 2.16 ± 0.12 m/s with 15, 22, and 37 N VC load (p < 0.001). Similar effects were observed during TVR activation with increases to 1.93 ± 0.13 m/s and 2.18 ± 0.14 m/s for low and moderate TVR amplitudes (p < 0.001). Increase of SWS at moderate TVR amplitudes correlated with that of 37 N VC load (r = 0.67, p < 0.001). TVR-induced stiffness changes at a 100 Hz vibration frequency and moderate amplitude may serve as a more objective alternative to VC-based loading for assessing functional muscle stiffness in asymptomatic individuals. However, clinical translation of TVR-THE as a standardized tool for assessing activation-related stiffness requires further evaluation in patient populations.

Background: Muscle activation is associated with increased tissue stiffness as measured by elastography in diagnostic applications. For this reason, we present ultrasound time-harmonic elastography (THE) applied to the vastus lateralis (VL) muscle during passive tonic vibration reflex (TVR) and active voluntary contraction (VC) stimulation to test whether TVR can serve as a stimulation method for functional assessment of skeletal muscle stiffness. Methods: Twenty-five asymptomatic volunteers (8 females, mean age 34 ± 8 years) underwent five consecutive THE examinations of the VL at three VC loads (15, 22, and 37 N) and during TVR stimulation with 100 Hz frequency and 200 µm (low) and 400 µm (moderate) amplitude. Using standard line-by-line ultrasound, THE acquired the induced shear waves of 60, 70, and 80 Hz frequency with a frame rate of 100 Hz. Shear wave speed (SWS) was reconstructed as a proxy for muscle stiffness and statistically analyzed with repeated-measures ANOVA and nonparametric Friedman tests. Results: SWS increased significantly from 1.66±0.11□m/s at rest to 1.79±0.12□m/s, 1.93±0.13 m/s, and 2.16±0.12 m/s with 15, 22 and 37 N VC load ( p < 10 ^-3 ). Similar effects were observed during TVR activation with increases to 1.93±0.13 m/s and 2.18±0.14 m/s for low and moderate TVR amplitudes ( p < 10 ^-3 ). Increase of SWS at moderate TVR amplitudes correlated with that of 37 N VC load ( r = 0.67, p < 10 ^-3 ). TVR-induced stiffness changes at 100 Hz vibration frequency and moderate amplitude can substitute the more subjective VC forces for muscle function testing. TVR stimulation combined with skeletal-muscle THE may be a useful tool for the routine clinical assessment of stiffness during muscle activation.

Purpose: Skeletal muscle is commonly modeled as a transverse isotropic medium, however, the behavior of its anisotropy under active loading remains insufficiently characterized. In this study we used ultrasound time-harmonic elastography (THE) to quantify direction-dependent shear-wave speed (SWS) in the vastus lateralis (VL) muscle at rest and during low isometric contraction intensities. Methods: Twenty-six healthy adults (15 men, 11 women; 25.0 ± 4.1 y) under-went multi-frequency THE (60–80 Hz). The transducer was aligned parallel (longitudinal) and perpendicular (transverse) to VL fascicles, and measurements were acquired at rest and at 15% and 30% of maximal voluntary contraction (MVC). The anisotropy index (AI) was defined as AI = SW S _∥ /SW S _⊥ . Orientation and contraction effects were tested with repeated-measures analyses. Results: At rest, longitudinal SWS exceeded transverse SWS ( 2.5 ± 0.2 vs. 1.4 ± 0.1 m/s; paired t-test p < 0.01 ). With contraction, SWS increased to 3.2 ± 0.2 and 3.8 ± 0.3 m/s (15%, 30% MVC) along fibers, and to 1.6 ± 0.1 and 1.8 ± 0.1 m/s across fibers (all p < 0.01 ). A two-factor repeated-measures ANOVA on SWS showed main effects of orientation and contraction and a significant interaction (all p < 0.01 ). AI increased from 1.7 ± 0.1 at rest to 2.0 ± 0.1 at 15% and 2.1 ± 0.1 at 30% MVC ( p < 0.01 ). No sex- or BMI-related effects were detected. Conclusion: VL exhibited marked shear-wave anisotropy at rest that increased with low-level contraction intensities, indicating disproportionate stiffening along the fiber direction. THE provides a rapid, cost-effective, orientation-sensitive readout of muscle mechanics that may support studies of neuromuscular function and pathology.

Purpose: Skeletal muscle is commonly modeled as a transversely isotropic medium; however, the behavior of its anisotropy under active loading remains insufficiently characterized. In this study, we used ultrasound time-harmonic elastography (THE) to quantify direction-dependent shear-wave speed (SWS) in the vastus lateralis (VL) muscle at rest and during low isometric contraction intensities. Methods: Twenty-six healthy adults (15 men, 11 women; $$25.0\pm 4.1$$ 25.0 ± 4.1 y) underwent multi-frequency THE (60–80 Hz). The transducer was aligned parallel (longitudinal) and perpendicular (transverse) to VL fascicles, and measurements were acquired at rest and at 15% and 30% of maximal voluntary contraction (MVC). Fractional anisotropy (FA) was computed from $$SWS_{\parallel }$$ S W S ‖ and $$SWS_{\perp }$$ S W S ⊥ , yielding zero for isotropic media. Orientation and contraction effects were tested with repeated-measures analyses. Results: At rest, longitudinal SWS exceeded transverse SWS ( $$2.5\pm 0.2$$ 2.5 ± 0.2 vs. $$1.4\pm 0.1$$ 1.4 ± 0.1 m/s; paired t-test $$p<0.01$$ p < 0.01 ). With contraction, SWS increased to $$3.2\pm 0.2$$ 3.2 ± 0.2 and $$3.8\pm 0.3$$ 3.8 ± 0.3 m/s (15%, 30% MVC) along fibers, and to $$1.6\pm 0.1$$ 1.6 ± 0.1 and $$1.8\pm 0.1$$ 1.8 ± 0.1 m/s across fibers (all $$p<0.01$$ p < 0.01 ). A two-factor repeated-measures ANOVA on SWS showed main effects of orientation and contraction and a significant interaction (all $$p<0.01$$ p < 0.01 ). FA increased from $$0.37\pm 0.04$$ 0.37 ± 0.04 at rest to $$0.43\pm 0.04$$ 0.43 ± 0.04 at 15% and $$0.47\pm 0.03$$ 0.47 ± 0.03 at 30% MVC ( $$p<0.01$$ p < 0.01 ). No sex- or BMI-related effects were detected. Conclusion: VL exhibited marked shear-wave anisotropy at rest that increased with low-level contraction intensities, indicating disproportionate stiffening along the fiber direction. THE provides a rapid, cost-effective, orientation-sensitive readout of muscle mechanics that may support studies of neuromuscular function and pathology.